Late Neoproterozoic carbonate productivity in a rifting context: the Adoudou Formation and its associated bimodal volcanism onlapping the western Saghro inlier, …

doi:10.1144/SP297.14 2008; v. 297; p. 285-302 Geological Society, London, Special Publications

and Magali Ader J. Javier Álvaro, Mélina Macouin, Hassan Ezzouhairi, A. Charif, N. Ait Ayad, M. Luisa Ribeiro

onlapping the western Saghro inlier, MoroccoAdoudou Formation and its associated bimodal volcanism Late Neoproterozoic carbonate productivity in a rifting context: the

Geological Society, London, Special Publications

serviceEmail alerting

article to receive free email alerts when new articles cite thisclick here

requestPermission to seek permission to re-use all or part of this article click here

SubscribePublications or the Lyell Collection

to subscribe to Geological Society, London, Specialclick here

Notes

Downloaded by on 3 June 2008

London © 2008 Geological Society of

http://sp.lyellcollection.org/cgi/alerts

http://sp.lyellcollection.org/cgi/reprintpermission?citation=%26Aacute%3Blvaro+et+al.+297+%281%29%3A+285

http://sp.lyellcollection.org/subscriptions

Late Neoproterozoic carbonate productivity in a rifting context:

the Adoudou Formation and its associated bimodal volcanism

onlapping the western Saghro inlier, Morocco

J. JAVIER ALVARO1,2, MELINA MACOUIN3, HASSAN EZZOUHAIRI4, A. CHARIF4,

N. AIT AYAD4, M. LUISA RIBEIRO5 & MAGALI ADER6

1Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spain2Current present address: UMR 8014-LP3, USTL, 59655 Villeneuve d’Ascq, France

(e-mail: [email protected])3UMR 5563-LMTG, 14 Av. Edouard Belin, Universite Paul Sabatier, 31400 Toulouse, France

4Departement de Geologie, Universite Chouaib Doukkali, 24000 El Jadida, Morocco5INETI—Area de Geociencias, Estrada da Portela, Zambujal, 2721-866 Alfragide, Portugal6UMR 7047, Institut de Physique du Globe de Paris, 4 place Jussieu, 75005 Paris, France

Abstract: An interval of episodic carbonate productivity, lithostratigraphically recognized as the‘Calcaires inferieurs’ (upper member of the Adoudou Formation), took place across the Neopro-terozoic–Cambrian transition onlapping the western Saghro inlier, Morocco. Sedimentation of the‘Calcaires inferieurs’ was highly variable: in relatively stable substrates, a peritidal-dominatedmixed platform is recorded where deposition was primarily controlled by autocyclic processesand accommodation space availability, whereas, in unstable substrates, the tectonic activityassociated with the inherited block-faulting basement led to deposition of complex slide sheetscomposed of penecontemporaneous isoclinal folds and disrupted strata. The uppermost part ofthe ‘Calcaires inferieurs’ displays a negative d13C shift reaching values of 26.5‰. This shiftmay represent the d13C excursion to 26‰ that marks the Neoproterozoic–Cambrian boundaryin the western Anti-Atlas. Two volcanic episodes bracketed the carbonate productivity. Theyconsist of lower basaltic flows and an upper rhyolitic ignimbrite, with a SiO2 gap between 52and 74 wt%. The basic rocks resemble those of tholeiitic magmas in continental rifts. Thefelsic rocks show high light to heavy rare earth element abundances and negative Nb, Ta, Pand Ti anomalies, and were probably generated as a result of either fractional crystallizationcoupled with relative crustal contamination, or from a different magmatic source. The lowerbasic flows of tholeiitic affinity predated and geochemically differ from the alkaline magmatismof the Alougoum volcanic complex (Boho jbel) that surrounds the neighbouring Bou-Azzer inlier.

Across the Neoproterozoic–Cambrian transition,the tropical climate, high CO2 content of the atmos-phere (Berger 1990) and low bathymetry werefavourable for high carbonate productivity patternsin some epeiric seas that bordered West Gondwana.Thus, thick and vast carbonate platforms formed,where a major replacement of benthic communitiestook place by the beginning of the Atdabanian (c. 20Ma after the Neoproterozoic–Cambrian boundary):a stromatolite-dominated consortium was replacedby shelly–metazoan and thromboid consortia(Alvaro et al. 2006a), which led in the MoroccanSouss Basin to the occurrence of the so-calledEarly Cambrian Great Atlasian Reef Complex(Alvaro & Clausen 2007).

Another major factor that controlled the estab-lishment and survival of carbonate factories acrossthe Neoproterozoic–Cambrian transition in the

Souss Basin (Morocco) was the configuration of abasement inherited from the Pan-African orogenyand the succeeding onset of an aborted rifting.Volcanoclastic input and tectonically inducedsubsidence affected dramatically the accommo-dation space and the geometry of a basin thatinherited a complex palaeotopographic basement.The basement of the Taroudant Group was primar-ily controlled by the superimposed effect of thePan-African orogeny and the onset of a complexnetwork of volcanic centres and volcano-sedimentary deposition (the Ouarzazate Group).Subsequent pulses of rifting reactivation modelledthe basin architecture and controlled the nucleationof centres of carbonate productivity (TaroudantGroup). To study the interplay between carbonateproductivity and the influence of inheritedcomposite palaeotopographies (Pan-African and

From: ENNIH, N. & LIEGEOIS, J.-P. (eds) The Boundaries of the West African Craton. Geological Society, London,Special Publications, 297, 285–302.DOI: 10.1144/SP297.14 0305-8719/08/$15.00 # The Geological Society of London 2008.

Ouarzazate deposition), synrift volcanism andtectonic instability, we have focused our study onthe characterization of the Adoudou volcanic andvolcano-sedimentary complex that onlaps thewestern margin of the Saghro inlier in the centralAnti-Atlas. This area offers a distinct palaeogeogra-phical configuration across the Neoproterozoic–Cambrian transition related to: (1) the activeerosion of the source area (the Saghro inlier)followed by the development of a brief episode ofcarbonate productivity; (2) the proximity to thesource area that induced a dramatic reduction incarbonate thickness and the distinct terrigenouscharacter of the overlying Lie-de-vin Formation(Tikirt Member); (3) the episodic instability of thecarbonate sea floor; and (4) the record of two volca-nic pulses that bracketed the carbonate productivity,different from the coeval alkaline magmatism of theAlougoum volcanic complex that surrounds theneighbouring Bou-Azzer inlier.

The purpose of this paper is threefold: (1) toanalyse the environmental and palaeogeographicalfactors that controlled the development of carbonateproductivity recorded in the ‘Calcaires inferieurs’that onlap the western Saghro inlier (Fig. 1); (2)

to propose a high-resolution sequence and chemo-stratigraphic framework for this onlapping, carbon-ate-dominated, depositional system; (3) to includethis stratigraphic sketch in a magmatic context bycharacterizing, both petrologically and geochemi-cally, the volcanic activity that bracketed thisepisode of carbonate productivity, and differentiat-ing it from neighbouring volcanic episodes. Thevolcanic activity in the Ait Saoun area consists oftwo lower basaltic and andesitic–basalt flows,interbedded in the upper part of the basal massiveconglomerates, and an upper acidic tuff cappingthe Adoudou Formation. Although the lower basicflows have been related to the activity of the Alou-goum volcanic complex (flanking the Bou-Azzerinlier), it represents a different volcanic episode asdemonstrated by its different geochemical affinity.

Geological setting and stratigraphy

The Souss Basin is one of the sedimentary troughsthat bordered the western Gondwanan marginduring early Palaeozoic times (Geyer & Landing1995). Infill of the Souss Basin unconformably

Fig. 1. (a) Regional map, and (b) geological map of the Saghro jbel and El Graara massif (central Anti-Atlas),showing locations of studied sections (modified from Service Geologique du Maroc 1970).

J. J. ALVARO ET AL.286

overlies the Proterozoic basement, and consists ofNeoproterozoic to Silurian (Late Carboniferousaccording to Helg et al. 2004) sediments. The base-ment of the Souss Basin was consolidated duringthe Pan-African orogeny (see the synthesis byGasquet et al. 2005). This was succeeded by depo-sition of thick volcano-sedimentary complexes,sometimes considered as late Pan-Africanmolasses, the so-called Saghro Supergroup (alsonamed Precambrian II3 or PII3), subsequently over-lain by the Ouarzazate (or PIII) Supergroup. Thelatter is also capped by a major unconformity (thecontact of the Ouarzazate–Taroudant groups),which displays both angular discordance andconformable contacts, probably related to normalfaulting and the growth and decay of volcaniccentres (Soulaimani et al. 2003, 2004).

After the Pan-African orogeny, broad shallowepeiric seas occupied extensive areas of the SoussBasin, which recorded a phase of intra-continentalextension. This led to development of a multi-steprifting, characterized by magmatism with tholeiiticand alkaline affinities, and a distinct diachroneityrelated to the rifting propagation from the Anti-Atlas to the western High Atlas and the Mesetadomains (Pique et al. 1995). Finally, activity onthis rift ceased in early Middle Cambrian times,

associated in the Anti-Atlas with a sharp intervalof uplift, erosion and karstification recorded at thetop of the Breche a Micmacca, and related to achange from active rifting to more passiveregional subsidence (Pique et al. 1995; Ait Ayadet al. 1998; Soulaimani et al. 2003; Alvaro &Clausen 2006, 2008).

The rifting-related volcanism recorded acrossthe Neoproterozoic–Cambrian transition in theintra-cratonic Souss Basin was accompanied bymarked asymmetric changes in sedimentary archi-tecture, development of complex onlapping geo-metries, and sharp lateral variations in carbonateproductivity. After the onset of the Ouarzazate–Taroudant unconformity, the subsequent base-levelrise resulted in deposition of a transgressive silici-clastic unit (Soulaimani et al. 2004), composed offluvial and alluvial conglomerates (the ‘BasalSeries’ of the Adoudou Formation; Fig. 2). Theunit passes upwards into a thick successiondominated by shallow-marine carbonates bearingscattered evaporitic pseudomorphs, and rich in stro-matolitic and thombolitic dolostones (‘Calcairesinferieurs’ of the Adoudou Formation andLie-de-vin Formation; Schmitt 1979). Althoughthe related carbonate productivity attained c.1000 m thickness in the central and distal parts of

Fig. 2. Stratigraphic summary of the Taroudant Group in the Anti-Atlas; after Boudda et al. (1979), Houzay(1979), Tucker (1986), Buggisch & Flugel (1988), Latham & Riding (1990), Kirshvink et al. (1991), Magaritz et al.(1991), Compston et al. (1992), Geyer & Landing (1995), Landing et al. (1998), Gasquet et al. (2005) andMaalouf et al. (2005); the stratigraphic interval studied here is boxed.

ADOUDOU VOLCANO-SEDIMENTARY COMPLEX 287

the Souss Basin (western Anti-Atlas), in the proxi-mal and marginal parts of the basin (central andeastern Anti-Atlas) the carbonate productivity wasrestricted by the influence of accommodationspace, neighbouring topographic relief and terri-genous input, and sometimes is absent (Destombeset al. 1985).

The Adoudou Formation (also known as Adou-dounian; Choubert 1952) forms the lower part ofthe Taroudant Group, and represents the oldestvolcano-sedimentary complex unconformablycovering the Neoproterozoic Ouarzazate Group(Fig. 2). The formation is currently divided intotwo members: the so-called ‘Basal Series’ or ‘Seriede Base’, less than 150 m thick, and the uppermember or ‘Calcaires inferieurs’, 50–1000 mthick. In the depocentre of the Adoudou Formation,located in the western Anti-Atlas, the ‘BasalSeries’ can be subdivided into three units: (1) thelower massive conglomerates (up to 150 m thick),dominated by massive conglomerates and secondarybreccias, sandstones and shales, locally interbeddedwith volcanic ashes and flows, which are widely dis-tributed throughout the Atlas Mountains overlyingthe Ouarzazate–Taroudant unconformity; (2) amiddle unit (up to 50 m thick in jbel Imider and200 m in the Anezi trough; Choubert 1952; Chazan1954), named ‘petit calcaire’, composed of dolos-tones, limestones and shales, and bearing phosphori-tic pockets and laminae in the High Atlas (Viland1977); (3) an upper shale-dominated unit (up to60 m thick; Boudda et al. 1979; Demange 1980).The upper member of the Adoudou Formation or‘Calcaires inferieurs’ is a monotonous successionof bedded and massive dolostones, locally inter-rupted by shales and sandstones. The base of the‘Calcaires inferieurs’ is characterized in thewestern Anti-Atlas by a dolostone unit, 50–200 mthick, partly silicified and locally mineralized bydiverse ores, named the Tamjout Bed (Chazan1954; Demange 1980). The occurrence of thickvariegated shale interbeds marks the transition tothe overlying Lie-de-vin Formation (also namedTaliwinian; Boudda et al. 1979).

The palaeogeographical distribution of thecarbonate factories related to the ‘Calcaires infer-ieurs’ also depended on the record of a continentalvolcanism. A striking volcanic activity is reportedfrom the central Anti-Atlas, where several trachyte,trachyandesite and basalt flows and numerouspyroclastic tuffs are intercalated. The centre of vol-canism has been conventionally associated with theAlougoum volcanic complex located in the El Gloaarea (Choubert 1952; Boudda et al. 1979; Fig. 2), inwhich volcanic palaeotopographies (such as theBoho volcano) directly cover the Adoudoudolostones and were progressively onlapped bythe breccias, dolostones, variegated shales and

sandstones of the Lie-de-vin Formation (Alvaroet al. 2006b). An early U/Pb date of 534 + 10Ma from the Boho volcano (Ducrot & Lancelot1977) has been recently refined to 531 + 5 Ma byGasquet et al. (2005). Three U/Pb dates are alsoavailable from the uppermost part of the AdoudouFormation and the overlying Lie-de-vin Formation:525 + 0.46, 522 + 2 and 521 + 7 Ma (Compstonet al. 1992; Landing et al. 1998; Maalouf et al.2005), all of them below the first appearance ofbiostratigraphically significant fossils, located inthe overlying Igoudine Formation and Atdabanianin age (Sdzuy 1978; Alvaro et al. 2006a). Thepalaeontological record of the Adoudou Formationis represented by the conspicuous presence ofmicrobial mats and stromatolites, and the spottyoccurrence of the red alga Kundatia in the uppermember (Buggisch et al. 1978; Buggisch &Heinitz 1984; Buggisch & Flugel 1988), and a poss-ible finding of medusoid imprints in the uppershales of the ‘Basal Series’ (Houazy 1979). TheNeoproterozoic–Cambrian boundary is tentativelylocated in the upper part of the Adoudou dolostones(lower Tifnout Member) based on a chemostrati-graphic peak of d13C correlatable with coeval suc-cessions in Siberia and South China (Tucker1986; Latham & Riding 1990; Kirshvink et al.1991; Magaritz et al. 1991).

Our study area is located at the westernmostedge of the Saghro inlier (Fig. 1), where two sec-tions of the ‘Calcaires inferieurs’ have beenstudied: they are located in the vicinity of IdBoukhtir (30 km to the WSW of Ouarzazate) andon the western flank of the Tizi n’Tinfifit jbel, inthe vicinity of Ait Saoun (38 km to the SSE of Ouar-zazate). There, the ‘Basal Series’ is composedexclusively of massive conglomerates and basiclava flows, the ‘Calcaires inferieurs’ are only 50–60 m thick (disappearing c. 15 km to the NE;Boudda et al. 1979), and the overlying Lie-de-vinFormation is represented by the Tikirt Member, amonotonous succession of reddish sandstonesand shales, devoid of carbonates, and up to 300 mthick. This paper documents the facies andsequence arrangement of the dolostones thatcharacterize the ‘Calcaires inferieurs’, and themineralogical and geochemical features of thebimodal volcanism that bracketed this reducedepisode of carbonate productivity. This consists oftwo lower basaltic and andesitic–basalt flows,interbedded in the upper part of the basal massiveconglomerates, and an upper acidic tuff cappingthe Adoudou Formation. Although the lower basicflows have been currently related to the activity ofthe Alougoum volcanic complex (located flankingthe Bou-Azzer inlier, 40 km to the SSW of thestudy area; Choubert & Faure-Muret 1970;Boudda et al. 1979; Fig. 2), it represents a different


volcanic episode, as demonstrated by its differentgeochemical affinity.

Facies and sequence arrangement of the

‘Calcaires inferieurs’ in the Ait Saoun area

Peritidal mixed (carbonate–siliciclastic)

facies association

The lower and upper parts of the ‘Calcaires infer-ieurs’ are characterized, in the Ait Saoun area(Fig. 1), by a heterogeneous lithology composedof carbonate and shale–litharenite on the bedformand bedset scale. Four major facies occur; theseare, in order of decreasing abundance: (1) varie-gated shale; (2) grainy dolostone; (3) fenestral,silty–cherty sparry dolostone; (4) stromatoliticdolostone (Figs 3 and 4a).

The intervals of purple and reddish shale arecommonly less than 3 m thick, and contain inter-bedded gravel-to-litharenite beds. The latter areup to 10 cm thick, and can be followed laterallymore than 10 m. Texture consists of a medium-sorted, subrounded to subangular, medium-grainedlitharenite dominated (.60% in volume) by felsicpyroclasts rich in euhedral feldspar (partially towholly replaced by albite and sericite), engulfedquartz, and accessory mafic phenocrysts (partlychloritized and illitized), and containing alsominor proportions of subrounded gravels of sparrydolostone, chert and shale intraclasts (Fig. 4b).This facies is either lithified by a calcareous matrixor poorly cemented. Sedimentary structures are high(c. 258) and low (c. 108) planar cross-lamination,low-amplitude (2–6 cm), wide (c. 50 cm) basalscours, and positive grading.

The grainy dolostone consists of thin- tomedium-bedded, coarsely crystalline strata, withmoderately preserved packstone to grainstone tex-tures (Fig. 4c and d). Skeletal grains are absent,and allochems are peloids, peloidal aggregates,millimetre-sized oncoids, quartz sand grains, anddolomite and shale intraclasts. Peloids are struc-tureless spheres of fine-grained dolomite thatoccur as single spheres and as flocculent coalescedclots or aggregates. Preservation of peloids andoncoids is locally poor and identification in thefield is difficult because of pervasive dolomitiza-tion. Cross and planar laminations are present butpoorly preserved. The base of the grainy dolostonebeds is commonly erosive, and covered by a lagdeposit (up to 30 cm thick), composed of contortedand distorted centimetre-thick beds of sparrydolostone, laterally fragmented into angularintraclasts (Fig. 4e and f).

Fenestral, silty–cherty dolostone strata are0.1–0.8 m thick, and display partially silicified

dolomicrosparite to dolopseudosparite textures.Chert nodules and layers, up to 10 cm thick, areconcentrated along the bedding planes, but rarelyform a continuous bed. Thin sections of the dolo-mite show disseminated, silt-sized, quartz andmica grains. Sedimentary structures include wavyand low-angle lamination, shaly partings, sharplower contacts, and transitional upper boundariesgrading into stromatolitic dolostones. The fenestralfabric is characterized by the presence of thinly dis-rupted laminae bearing abundant, millimetre-sized,spar-filled fenestrae and vugs, or consists of thicklayers disrupted by discontinuously bedded,spar-filled fenestrae.

The stromatolitic dolostone consists of a basalunit (0.1–0.4 m thick), which comprises fenestraland silty dolomite couplets, and an upper unit (upto 1.4 m thick) composed of microbial flat, wavyor crinkled microbial laminites capped by a con-tinuous package of decimetre-thick, dome-shapedstromatolitic mounds. The silty dolomite has sharpbases that are, in some cases, lined by flat pebbleintraclastic lags. The stromatolitic fabric is punctu-ated with stylolitic and argillaceous seams, wherechert nodules and laminae also occur (Fig. 4g).Stromatolitic laminae are characterized by subtlechanges in the size of dolomite crystals, and areoccasionally delineated by either a slight increasein abundance of quartz silt grains or chertylaminae. Stringers of quartz silt, vugs, and chertylaminae also delineate lamination in the intermoundfill. Domal stromatolites form unlinked mounds,with up to 50 cm synoptic relief, grading laterallyinto low-relief domal stromatolites or wavymicrobial dolostone. Stromatolitic heads are com-monly separated by c. 10–50 cm wide areas filledwith silty dolomitic packstone that forms an anasto-mosing and laterally migrating pattern betweenmounds. At the top of the domal stromatolites,they are smoothly laminated, have low synopticrelief, and are irregularly cracked. Graded, cross-laminated dolomitic grainstone (similar to theaforementioned grainy dolostone) fills the reliefbetween the heads of the uppermost dome-shapedstromatolites and the centimetre-scale cracks.

The episodic input of litharenite interbedssuggests a local pyroclastic source. The lower peloi-dal dolopackstone to grainstone represents subtidalto possible intertidal peloidal sheets (or low-angleshoals) with an intermittent pyroclastic flux. Sharpcontacts and conglomeratic rip-ups in the base ofsome beds suggest storm energy sufficient todisrupt the sea floor. The depositional environmentof the coarse-crystalline dolostone is not clear butthe minor interbeds of the nodular, peloidalmudstone to packstone may suggest a subtidalsetting. An agitated shallow subtidal environmentis envisaged for the stromatolite-dominated parts.


The high synoptic relief of the stromatolites andthe lack of distinct subaerial exposure surfaceswithin associated sediments indicate that microbialmounds accreted in a wave-dominated subtidalsetting, a view supported by the high-energy grain-stone fill between mounds.

Offshore dolomite–shale rhythmites

This second facies association is located in themiddle part of the ‘Calcaires inferieurs’ (Fig. 3)and is monotonously laminated. Muddy and micro-sparitic dolomite laminae are interlaminated withreddish and purple shale. The facies is for themost part monotonously laminated, with localdevelopment of cross-laminated siltstone lenses,2–5 cm thick, and lenticular-to-flaser fabricslacking erosive contacts (Fig. 4h). Microspariticdolomite is light red, massive to thinly bedded,and has moderately well-preserved mudstone

textures. Sedimentary structures include planebeds, wavy lamination, thin laminae of clay, andthin beds of siltstone.

Because of the scarcity of erosive and currentsedimentary structures, these rhythmites weredeposited on a calm substrate, which rhythmicallyrecorded a fine-grained siliciclastic influx trans-ported via decantation, and was episodicallyaffected by distal storm events below fair-weatherwavebase.

Cycles and controls

The aforementioned peritidal facies association isarranged into 1.5–5 m thick cycles or parase-quences (Fig. 3). The cycle boundaries of thesemixed (carbonate–siliciclastic) cycles are recog-nized on the basis of erosive surfaces located atsharp lithological contacts followed by sharpdeepening. The cycles begin with (1) purple and

Fig. 3. Stratigraphic logs of the ‘Calcaires inferieurs’ from the Ait Saoun and Id Boukhtir sections. (a) Legend.(b) d13C‰ PDB isotopic data from calcite and dolomite. (c) d13C/ d18O crossplot of dolostone samples.(d) Arrangement of facies associations, cycles and chemostratigraphic variations.


reddish shale with interbedded, gravel-to-litharenite,pyroclastic event beds, (2) grainy (peloidal-dominated grainstone to packstone) and fenestral,silty–cherty dolostone, and (3) wavy-to-crinkledmicrobial dolostone, which passes upwards intodomal stromatolites. The top of the stromatolitic

domes commonly contains cracks and fissures(crosscutting a fully rigid sediment), which arefilled and covered by (4) peloidal–intraclastic dolo-grainstone–rudstone and fenestral dolostone,common also in the lows between stromatoliticdomes. Larger stromatolites locally pass laterally

Fig. 3. Continued.


Fig. 4. (a) Panorama of the western flank of the Tizi n’Tinfifit jbel (Ait Saoun) with arrowed base and top of the‘Calcaires inferieurs’ Member; b, basalt; ab, andesitic basalt. (b) Volcanoclastic limestone showing clasts, composed ofpartly silicified (left rim) mudstone (mud), chert (ch) and felsic debris (fel), embedded in a micritic matrix (mi);scale bar represents 1 mm. (c) Contorted beds of sparry dolostone and breccia intraclasts at the base of theperitidal mixed cycles; erosive surfaces are arrowed, the uppermost marking the contact with the overlyingpeloidal-dominated grainstone; scale bar represents 4 cm. (d) Lateral transition from the previous view, showingabundant dolostone clips and the same upper contact (arrowed); scale bar represents 10 cm. (e) Peloidal andoncoidal grainstone encrusted by a microbial film (arrowed), subsequently covered by a volcanoclastic lag; scalebar represents 1 mm. (f) Detail of peloidal and oncoidal grainstone showing the perfect preservation of microbialtextures despite dolomitization and neomorphism; scale bar represents 500 mm. (g) Stromatolitic dolostone(d) punctuated by silicified strings (s); scale bar represents 1 mm. (h) Reddish, microsparite dolomite–shalerhythmites showing parallel and low-angle lamination, and some contorted contacts where the shales are thicker.


into smaller stromatolites, peloid-rich microbialdolomite and intraclastic rudstone.

The lower part of the shallowing-upward(coarsening-upward) cycles may record a shoalingdepositional sequence, with interbedded shalesand dolostones giving way to peloidal-dominated,grainstone low-angle shoals and sheets, indicatingenvironments of increasing energy. This lowerpart of the cycle is capped by fenestral to stromato-lite mounds where microbial laminae pass verticallyfrom flat, crinkled and wavy into dome-shaped. Thestromatolitic mounds are capped by cracked sur-faces, which may reflect subaerial exposure andtepee structures, filled by dolomitic grainstone andrudstone. Efforts to trace cycles in the availableoutcrop showed that they are extremely variable,although they cannot be traced laterally morethan 200 m, and component facies pinch out orinterfinger with the other facies. The poor traceabil-ity of these peritidal cycles suggests an autogenicorigin. Overall, carbonate production and accu-mulation in the peritidal complex was sufficientto track sea-level fluctuations, so it can be con-sidered that the environment remained shallow(Pratt et al. 1992).

On a larger scale, the cycles are stacked in atransgressive-to-regressive depositional systemprimarily controlled by the availability of accom-modation space. The depositional system islimited, at its bottom and top, by two regressivecoarse-grained siliciclastic deposits: the ‘BasalSeries’ and the Tikirt members. In addition, thetransgressive and regressive trends of the deposi-tional system are separated by the aforementionedmixed rhythmites, which represent a major floodingof the platform (Fig. 3).

Facies and sedimentary geometry of the

‘Calcaires inferieurs’ at Id Boukhtir

At Id Boukhtir, the strata of the ‘Calcaires infer-ieurs’ (Van Looy 1985) lack distinct cycles, as aresult of the presence of numerous erosive trunca-tions (Fig. 5a). A suite of c. 58 m of stromatoliticdolostones (Fig. 5b) and local litharenites isunderlain and internally interrupted by successivesedimentary unconformities (Fig. 3). These areinterpreted as large-scale, sliding flows where car-bonate strata are commonly disrupted and slump

Fig. 5. (a) Panorama of the Id Boukhtir section taken from the Tazenakht–Ouarzazate road; the base and top of theslide sheet described in the text are arrowed. (b) Dome-shaped stromatolite unaffected by sliding. (c) Conical foldsmade up of wavy-to-crinkled stromatolites; hinge points are arrowed; width of compass is 7 cm. (d) Schmidt net plotof the crest lines of the measured cylindrical and conical synsedimentary folds, some of them bearing crestlineculminations and depressions, and their setting after correcting the stratification plane to the horizontal.


folded, on a small and medium scale, into disharmo-nic tight and isoclinal folds, with cylindrical andconical shapes (Fig. 5c). Their fold axes and crestlines trend c. 340–3508, after correction for tectonicdip (Fig. 5d), which is broadly parallel to the associ-ated inlier border. These geometries fit Schlische’s(1992) concept of ‘drag fold’. Lineations and foli-ations, which are commonly roughly axial planarto folds (Fig. 5c), are also observed in the hingesof slump folds. Models that attempt to explain thegenesis of penecontemporaneous foliation involvesimple load compaction superimposed on pre-existing folds or rotation of grains facilitated byslump folding (Tobisch 1984).

The presence of these gravity-related sedimen-tary structures (abundant in other laterally equival-ent strata; Buggsich & Heinitz 1984) suggests thatthe ‘Calcaires inferieurs’ were deposited in thisarea in an unstable slope environment, wheregravity movements disturbed the autochthonous(microbially dominated) reef sedimentation. As awhole, this sedimentary system can be interpretedas a large, plastically deformed olistolith or slidesheet of semi-consolidated dolostones (see the geo-metry shown in Fig. 5a). According to the averagefold axis orientation, the sea floor of the Id Boukhtirsection was a SW-facing slope bordering thewestern Saghro inlier basement. The pronounceddifference in facies between the Ait Saoun and IdBoukhtir sections, separated by only c. 50 km, indi-cates the development of a steep-sloped, possiblyfaulted-bounded, intra-shelf basin.

Geochemical methods

We analysed for chemostratigraphic purposes 24samples for carbonate d13C and d18O spanning amixed stratigraphic interval of 50 m from the‘Calcaires inferieurs’, to test the occurrence of cor-relatable shifts in d13C (Fig. 3). Between 1 and10 mg of powdered samples were reacted withH3PO4 at 25 8C for 18 h to extract the CO2 fromthe calcite, and then at 80 8C for 2 h to extract theCO2 from the dolomite (the method has beendescribed in detail by Swart et al. 1991), althougha perfect chemical separation of calcite anddolomite is difficult (Al-Assam et al. 1990; Yui &Gong 2003). The amount of extracted CO2 andits carbon and oxygen isotopic compositions weremeasured using a helium continuous-flow massspectrometer (AP-2003) at the IPGP laboratory(Paris). The reproducibility of the d13C andthe d18O measurements is +0.1‰ and +0.2‰,respectively.

Geochemical data from igneous rocks are basedon nine chemical analyses of representative samples(Table 1). Major, trace and rare earth elements

were determined using X-ray fluorescence andinductively coupled plasma mass spectrometry(ICP-MS) at the INETI laboratory in Porto. Pre-cision for major and trace elements is usuallybetter than 2% and 5–10%, respectively.

Chemostratigraphic control

Recently, Maalouf et al. (2005) have updated andproposed a composite d13C curve for the Neopro-terozoic–Cambrian transition in Morocco. Theydefined two members for the Adoudou Formationin the western Anti-Atlas, named the Tabia andTifnout members. However, the presence of periti-dal carbonates in both members makes difficult todifferentiate them in the central and eastern Anti-Atlas, where Maalouf et al. identified only theTabia Member. The stratigraphically condensedcharacter of the ‘Calcaires inferieurs’ borderingthe western Saghro inlier is probably due to lowsedimentation rates and not to sharp erosive sur-faces, absent in the facies associations describedabove. This can prevent the identification of theTabia–Tifnout contact in the study area, which isa key marker bed for chemostratigraphic corre-lation. The latter is based on a d

13C excursion to26‰ at the base of the Tifnout Member (Fig. 2)that is correlated with the Neoproterozoic–Cambrian boundary in Siberia and Death Valley,and dated at 542 + 0.6 Ma in Oman by Amthoret al. (2003) and Maalouf et al. (2005). A positived13C shift of 7‰ located in the upper part of theTifnout Member was proposed by the sameworkers as the Nemakit–Daldinian–Tommotianboundary (Early Cambrian; Fig. 2).

d18O values vary between 22.3 and 28.7‰ andare in accordance with those obtained by Maaloufet al. (2005). When plotting the d13Ccarbonate as afunction of the d18Ocarbonate (Fig. 3c), the lack of cor-relation between d13C and d18O indicates that thesesamples have not been strongly modified by meteo-ric diagenesis. In addition, the profiles of d13Ccalcite

and d13Cdolomite are parallel, reflecting the lack ofdiagenetic processes affecting preferentially thed13C values in calcite or dolomite. The d13Cprofile is not correlatable with facies or with parase-quence trends. d13C background values for thissection gradually increase (from 22.4 or 21.3 to20.6 or þ0.4‰) and decrease to broadly constantvalues ranging from 24 to 22‰, as shown in thelower and middle parts of the section. However,this trend is interrupted in the upper part of thesection by two sudden negative shifts of d13Cvalues to c. 21.3 to 21.8‰ in the lower, andc. 23.2 to 23.6% in the upper part. After bothnegative shifts, the values sharply decrease tobackground averages.


In the Ait Saoun area, the upper negative d13Cshift reaches values of 26.4 or 26.6‰, whichmay represent the d13C excursion to 26‰ locatedat the base of the Tifnout Member and proposedas the Neoproterozoic–Cambrian boundary inthe western Anti-Atlas (Maalouf et al. 2005).However, the lack of carbonate strata in theoverlying regressive Tikirt Member precludesany possible identification of the succeedingevolution of the d13C values in this proximal area.In addition, the associated d18O values (22.3 and23.7‰ for d18Ocalcite and d18Odolomite, respect-ively) appear higher than in others samples (23.2to 28.1‰ and 25.3 to 28.7‰ for d18Ocalcite and

d18Odolomite, respectively) displaying similar litho-logies and facies, and might indicate a diageneticsignal. As a result, our chemostratigraphic corre-lation of the Neoproterozoic–Cambrian boundaryin the Ait Saoun area is only tentatively proposed.

Mineralogical and geochemical features

of the associated volcanism

As explained above, an episodic volcanic activitywas coeval in the study area with carbonate pro-ductivity across the Neoproterozoic–Cambrian tran-sition. This is documented by the presence of felsic

Table 1. Selected chemical analyses from the basalts, andesitic basalts and rhyolitessampled in the Ait Saoun area

Sample: AS-1 AS-2 AS-3 AS-4 AS-5 AS-6 Ouri1 AS-12 AS-13

(wt %)SiO2 46.08 45.89 46.63 51.8 49.64 50.22 73.75 75.63 78.26Al2O3 15.02 15.12 14.23 13.43 13.43 13.91 10.63 9.14 8.38Fe2O3* 15.84 15.6 17.25 12.19 13.19 13.35 2.99 2.62 2.35MnO 0.11 0.17 0.15 0.1 0.09 0.08 0.45 0.06 0.04MgO 8.15 5.69 6.06 4.82 5.59 8.79 0.55 0.21 0.31CaO 2.93 4.53 3.39 2.81 2.97 1.39 0.4 1.61 1.26Na2O 3.67 3.73 4.4 3.03 2.98 2.15 0.13 0.33 0.2K2O 0.8 1.73 1.27 3.83 3.48 3.28 8.54 7.91 7.1TiO2 3.53 3.36 3.26 3.1 3.11 3.31 0.45 0.49 0.46P2O5 0.34 0.34 0.33 0.64 0.65 0.68 0.08 0.12 0.13LOI 3.08 3.24 2.6 4.02 4.64 2.38 1.37 1.66 1.43Total 99.55 99.4 99.57 99.77 99.77 99.54 99.36 99.78 99.92

( ppm)Rb 17 42 21 41 37 30 83 59 55Sr 56 61 96 107 91 58 35 19 20Y 38 38 35 63 66 66 23 13 12Zr 143 144 137 285 288 283 153 133 129Nb 4 5 5 11 11 11 7 5 4Ba 101 73 161 617 438 287 1357 497 489Ta 0.24 0.3 0.3 0.66 0.66 0.66 0.74 0.3 0.24Th 13.8 6 6Hf 4.29 4.32 4.11 8.55 8.64 8.49 4.56 3.99 3.87La 13 10.7 9.4 21.9 25.1 23 20 7 14Ce 30.4 26.4 24.2 56.1 60.9 59.2 43.9 33 21Pr 4.6 4.2 3.9 8.2 8.9 8.6 5.2Nd 24.4 21.3 19.3 39.9 42.6 41.4 21.1 13 7Sm 6.2 5.8 5.2 10.2 10.7 10.6 4.7 2 2Eu 2.5 2.3 1.9 3.4 3.8 3.2 1.3Gd 7 6.6 5.8 11.2 11.5 11.9 4Tb 1.1 1.1 1 1.8 1.9 1.9 0.7Dy 6.6 6.5 6.1 10.9 11.2 11.5 4.1Ho 1.4 1.3 1.3 2.3 2.3 2.4 0.9Er 3.6 3.6 3.4 6.2 6.3 6.6 2.4Tm 0.5 0.5 0.5 0.9 0.9 1 0.4Yb 3.4 3.3 3.2 5.8 5.9 6.3 2.4 ,6 ,6Lu 0.5 0.5 0.5 0.9 0.9 1 0.4

*Total iron as Fe2O3.LOI, loss on ignition.


ash encased in carbonates. However, the carbonateproductivity recorded in the Adoudou volcano-sedimentary complex was bracketed between twomajor episodes of effusive and pyroclastic activity:a lower basic episode interbedded within the upper-most coarse-grained siliciclastic strata of the ‘BasalSeries’, and an upper acidic episode located at theAdoudou–Lie-de-vin contact. Field, petrological

and geochemical analyses were undertaken toimprove understanding of the eruptive style ofthese volcanic rocks to provide insight into theirenvironment of deposition.

The first basic volcanic episode is representedby two lava flows that occur interbedded in theuppermost part of the ‘Basal Series’ (Figs 4a and6b). This member displays sharp changes in

Fig. 6. (a) Conformable contact of the basal conglomerates (Cg) and the basal flow (B). (b) Thin-sectionphotomicrograph of the basalt lava showing its microlithic to slightly porphyritic texture; polarized light; scale barrepresents 2 mm. (c) Breccia formed at the basaltic lava–red sandstone contact. (d) Thin-section photomicrographof the andesitic basalt displaying a microlithic texture (note the presence of quartz floating in the groundmass); cross-polarized light; scale bar represents 1 mm. (e) Ignimbritic rhyolite capping the Adoudou Formation. (f) Thin-sectionphotomicrograph of the rhyolitic tuff showing a felsic texture; scale bar represents 0.4 mm. Chl, chlorite; FeMg,ferromagnesian minerals; Pl, plagioclase; Q, quartz; R, rhyolite; stro, stromatolitic dolostone.


thickness, reaching 80 m to the east of Ait Saoun,where the contact is marked by a NNE–SSW-striking fault, which probably acted as anormal fault coevally with deposition of the‘Basal Series’. In the study area, interflow strataconsist of centimetre-bedded, siltstone-to-conglo-merate wedge-shaped units. Laterally, the lavaflow changes sharply into a matrix-supportedbreccia. The lower lava flow is vesicular, darkgrey basalt, 10–15 m thick. It has a microlithictexture, locally changing to porphyritic. The micro-phenocrysts are predominantly euhedral plagioclase(�2 mm in size), and highly altered femic minerals(mainly olivine and pyroxene), widely replaced byan association of chlorite, epidote, calcite, andiron oxide. The groundmass is rich in plagioclaseand iron oxide microliths (Fig. 6b). Flow tops arevesicular, where irregular centimetre-sized amyg-dales are occluded by chlorite and calcite. Thislower lava flow is overlain by a breccia-like bed(1–3 m thick) that displays a mixture of volcanicpockets, with indistinct outlines, and fine- to coarse-grained litharenites (Fig. 6c), reflecting the partialreworking of the lava embedded in a coarse-grainedsiliciclastic sea floor. The upper lava flow, c. 15 mthick, is grey andesitic basalt flow with a vacuolarmicrolithic texture. The microphenocrysts consistof plagioclase (c. 1 mm long), femic minerals(less abundant than in the lower basalt flow and gen-erally altered), and microliths of iron oxide, feld-spar and rare tiny quartz (Fig. 6d). The top of thislava flow is also overlain by another ‘breccia’ ofdecimetre scale, composed of magmatic pocketsand litharenites.

The younger acidic volcanic episode, whichmarks the Adoudou–Lie-de-vin contact, is only0.2–0.5 m thick and has up to 200 m lateralextent (Fig. 6e). Although the thickness andlateral extent of this ignimbritic rhyolite, whichchanges laterally into a pyroclastic tuff, mayinitially appear relatively insignificant, they mayyield clues to the understanding of the formationof siliceous magmas. Petrologically, the ignimbriticrhyolite shows a fine equigranular felsic texture(Fig. 6f), where the phenocrysts are composed ofalbite with polysynthetic twins, K-feldspar, quartz,intergranular iron oxide, and subsidiary chlorite.Carbonate and siltstone clasts, derived from theunderlying strata, are also present.

From the geochemical compositions, three rocktype were distinguished in the volcanic episodes ofthe Adoudou volcano–sedimentary complex: basalts(45% , SiO2 , 47% and 15.5% , Fe2O3 , 17%),andesitic basalts (49% , SiO2 , 52% and 12%, Fe2O3 , 13.5%), and rhyolites (SiO2 . 74%)(see Table 1).

The Harker diagram representation of the avail-able data displays a hiatus on the intermediate field

(Fig. 7). The chondrite-normalized rare earthelements (REE) projection of basalts and andesiticbasalts displays parallel profiles suggesting theirdifferentiation from a common source; in contrast,the profile of the acidic rocks crosscuts the others,suggesting their provenance from a differentsource (Fig. 8). All the rock types display enrich-ment in light REE (LREE), which is greater in therhyolites (La/Yb ¼ 8.5) despite their low totalamount. The Y/Nb variations between 2.5 and 9.5

Fig. 7. Harker diagrams (Harker 1909) (in wt%) for thetwo volcanic episodes recorded in the Ait Saoun area.S, Basalt; þ, ignimbritic rhyolite.

10

100

200

La

Ce

Pr

Nd Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Sam

ple/

C1

Cho

ndri

te

++

++

+

++ +

+ + + + + +

Fig. 8. C1 chondrite-normalized (Sun & McDonough1989) REE patterns from the basalt flow (S), andesiticbasalt flow (S) and rhyolitic ignimbrite (þ) found in theAit Saoun area.


indicate that these rocks belong to the sub-alkalinetype. The projection of the basic rocks on Miyashiro& Shido (1975) and Pearce & Gale (1977) diagramsindicates they are intra-plate tholeiites (Fig. 9).The Zr/Hf, Ti/Zr and Nb/La ratios (33, 64–140and 0.31–0.53, respectively) are very close tothose reported for continental tholeiites (Dupuy& Dostal 1984; Marsh 1987). The spider diagramof these rocks is also similar to that for the SabieRiver and Lesotho (South Africa) continentaltholeiites (Marsh 1987) (Fig. 10). The Nb negativeanomaly, visible in all the profiles, is interpreted asindicative of crustal contamination (Bertrand 1975;Wood 1980; Dupuy & Dostal 1984; Wilson 1994).

The volcanism that brackets the establishmentand demise of carbonate factories (‘Calcaires infer-ieurs’) in the study area is bimodal, starting withlarge volumes of effusive basaltic lava flows andending with a reduced effusive and explosive, rhyo-litic volcanism. Intermediate compositions areabsent, as the major element and SiO2 content isdominated by basic and acidic end-members(Fig. 7). The present geochemical data do notpermit us to propose any linkage between thebasic and acidic volcanic episodes. Crustal con-tamination is obvious in both episodes, mostly inthe acidic volcanism. The two episodes may rep-resent either the extreme poles of a geochemicalfractionation derived from a common magma withstrong crustal contamination, or may have resultedfrom different sources with a crustal source forthe acidic episode. In this case, the basaltic mag-matic chamber could have contributed to melt thecrustal source of the acidic episode. Thebasic-to-acidic rock differentiation is a commonprocess in continental tholeiitic settings (Dostalet al. 1983; Marsh 1987; Wilson 1994).

As explained in the ‘Geological setting’ section,the volcanism was considered as being associatedwith that preserved in the Boho volcano (from theAlougoum volcanic complex; Fig. 2). However, thelatter shows alkaline affinities and evolved, viacrystal fractionation, from basic to intermediate andacidic (Alvaro et al. 2006b). The basic tholeiiticvolcanism of the Ait Saoun area is different fromboth the late Neoproterozoic K–calc-alkalinemagmatism recorded in the Ouarzazate Supergroup

250 500 750

3

5

Ti/Y

Zr / Y

Plate marginbasalt Within-plate

basalt

2 4 6 8

4

8

12

16

FeO*/MgO

FeO*

(a) (b)

TholeiiticDomainTh. + C. alk

Calc-alkalineDomain

Fig. 9. Geochemical affinity of the Ait-Saoun basalts according to (a) Miyashiro & Shido (1975)and (b) Pearce & Gale (1977) diagrams.

Fig. 10. MORB-normalized (Pearce 1983) traceelement patterns for representative basalts (S) andrhyolites (þ) of the Ait Saoun area, compared withcontinental tholeiitic basalts from Lesotho (B) and theSabie River (†), South Africa (after Marsh 1987), andalkaline basalt from Boho Jbel (O) (Alvaro et al. 2006b).


from the Ouarzazate–Agdz region (Ezzouhairi 2001;Ezzouhairi et al. 2008), and the alkaline magmatismof the neighbouring Jbel Boho (Bou-Azzer inlier).

Palaeogeographical and geodynamic

implications

All along the margins of the western Saghro inlier,and postdating the unconformity located at theOuarzazate–Taroudant contact, variations in thick-ness, facies and development of stratigraphic dis-continuities within the ‘Basal Series’ reflect thesyndepositional activity and along-strike variationsin displacement of intrabasinal faults (Schlische1992). Fault scarps along the basin margins hadoffsets of tens of metres, and remained significantmorphological escarpments on the basin floorduring deposition of the ‘Basal Series’. Increasedsediment supply outpaced the subsidence rate, andthe palaeorelief was finally levelled, favouringnucleation of carbonate factories. Much of thisrelief was reduced by the accumulation of alluvialand fluvial sediments (‘Basal Series’) intopographic lows prior to transgression andestablishment of carbonate factories (‘Calcairesinferieurs’). Main tensional and transtensionalfaults arranged the depocentres in a block-faultingbasement system and determined the regional dipof the onlapping carbonate strata.

The differential tilted fault-block setting of theAit Saoun and Id Boukhtir sections that flank thewestern Saghro inlier had important consequencesfor sequence-stratigraphic interpretations of theoverlying ‘Calcaires inferieurs’ because of asym-metric subsidence, which caused drastic changesin bounding surface character and sequence thick-ness. Sedimentation of the ‘Calcaires inferieurs’ inrelatively stable margins (e.g. Ait Saoun) isorganized in distinct peritidal cycles, separated bysurfaces of erosion and sharp deepening, whereasunstable sea floors (e.g. Id Boukthir) recordednon-cyclic intervals dominated by slope-relateddeposits. Tectonism and sediment supply (bothfrom pyroclastic and source areas) were the maincontrols on accommodation-space fluctuations andthe stratigraphic architecture of the marginssurrounding the western Saghro inlier. Othermixed platforms bordering neighbouring inliers(such as the Bou-Azzer inlier; Fig. 1) recordedsimilar peritidal environments and low thicknessof the ‘Calcaires inferieurs’, rich in stromatolites,sheet-cracks, fenestrae and evaporitic pseudo-morphs (Chbani et al. 1999). Finally, the prograd-ing Tikirt Member that overlies the AdoudouFormation westwards and northwards may berelated to either a sea-level fall, not clearly recog-nized in distal areas (western Anti-Atlas), or

tilting and flexuring of proximal blocks of theAnti-Atlas margin (Maalouf et al. 2005). Thelatter, also related to volcanic acidic activity,would lead to enlargement of the emerged sourceareas, resulting in increased sediment supply andthereby feeding and favouring the progradation ofthe Tikirt sedimentary system.

The geochemical features of the bimodal vol-canism (with large amounts of basic volcanic lavaand reduced rhyolite tuffs) described in the studyarea testify to the initial rifting stages of a thickenedcrust similar to the continental tholeiites of Lesothoand the Sabie River in South Africa. The beginningof this rifting took place after the Pan-Africanorogeny, as documented by the K–calc-alkalinevolcanism recorded in the Ouarzazate Supergroup(or Precambrian III; Ezzouhairi 2001), and wasreactivated after the unconformity that marks theOuarzazate–Taroudant contact (Pique et al. 1995;Soulaimani 2001, 2003, 2004). Discontinuity sur-faces, and the thickness and sedimentary characterof the ‘Basal Series’ molasses, vary greatly depend-ing on their location within the inherited palaeo-topography, because of an asymmetric subsidencepattern of tilted fault-blocks associated with awidespread extensional fault and volcanic activity(e.g. Benssaou & Hamoumi 2001; Ezzouhairiet al. 2003; Benssaou 2005). Across the Neoprotero-zoic–Cambrian transition, the partial burial ofthe inherited palaeotopography coeval with awidespread transgression favoured development inthe study area of carbonate productivity, whichrecorded the input of numerous felsic pyroclasts.The abundance of shard-rich pyroclasts indicatesthat much of the pyroclast debris was derivedfrom a neighbouring felsic subaerial or shallow sub-aqueous explosive volcanism. The final demise anddisappearance of the carbonate productivity is alsoassociated with the accentuation of an intermittentexplosive acidic volcanism and the establishmentof regressive conditions indicated by the prograda-tion of the overlying Tikirt Member.

These chemostratigraphic data can also be com-pared with Maalouf et al.’s (2005) sections 13–15from the El Graara inlier. These are located atsimilar easterly positions in the basin, and arecharacterized by similar stratigraphic frameworks(e.g. presence of a bimodal volcanism, autobrec-cias, and peritidal carbonates). In all of the ElGraara sections, d

13C values rise upsection from alow of 24‰ and then oscillate between 22‰and þ4‰, mimicking the isotopic profile of theTifnout Member in the western Anti-Atlas.However, at Ait Saoun, d13C drops rapidly from0‰ to 24‰, and then essentially remains at23‰ to 24‰ for the remaining 40 m of thesection. A correlation of this drop in d13C withthe Neoproterozoic–Cambrian boundary isotopic


shift (possibly observed in the upper part of theTifnout Member in the western Anti-Atlas) wouldsuggest that the basic volcanism that onlaps thewestern Saghro inlier is older than 542 Ma (seeFig. 2). This strengthens the lithostratigraphicargument (the basic lavas occur embedded in theuppermost part of the Basal Series) that theSaghro and El Graara volcanism are of differentage, and that accommodation space in the easternregions of the Anti-Atlas margin was also con-trolled by block faulting and thermal subsidencein the vicinity of volcanic sources.

Conclusions

The western margin of the Saghro inlier in thecentral Anti-Atlas is an example of a mixed platformwith low carbonate production rates and accumu-lation space. There, the establishment, demise andstyle of carbonate facies were directly influencedby the neighbouring post-Pan-African palaeoreliefand coeval volcanic activity. Succeeding the Ouar-zazate–Taroudant unconformity and sedimentationof stacked alluvial and fluvial conglomerates in thelows of the study area, carbonate deposition startedduring the following transgression, as evidencedby their related onlapping geometries. The faciesassociations and strata arrangement of the ‘Calcairesinferieurs’ in relatively stable substrates (Ait Saounsection) reflect both autocyclic and accommodation-space controls on a peritidal-dominated mixed plat-form. Carbonate factories were in part microbiallyinduced, as evidenced by the widespread develop-ment of stromatolitic mats and mounds. In contrast,the instability of the platform related to the tectonicactivity associated with the inherited block-faultingbasement is illustrated at Id Boukhtir, wherethe stromatolite-dominated ‘Calcaires inferieurs’form complex slide sheets composed of penecon-temporaneous isoclinal folds and a disrupted strati-fication. The uppermost interbedded dolostones ofthe ‘Calcaires inferieurs’ display in the Ait Saounarea a negative d

13C shift reaching values of 26.4to 26.6‰. This shift may represent the d13Cexcursion to 26‰ located at the base of theTifnout Member (Adoudou Formation), whichmarks the Neoproterozoic–Cambrian boundary inthe western Anti-Atlas.

The episode of carbonate productivity rep-resented by the ‘Calcaires inferieurs’ was bracketedbetween two episodes of tholeiitic volcanism. In thestudy area, the synrift volcanism was bimodal incharacter, and comprises two lower basaltic andandesitic basalt flows and upper ignimbritic rhyo-lites, with a SiO2 gap between 52 and 74 wt%.The basic rocks resemble those of the tholeiiticmagmas in continental rifts. The felsic acidicrocks show high large ion lithophite element

abundances and negative Nb, Ta, P and Tianomalies, and were probably generated either asa result of fractional crystallization coupled withrelative crustal contamination, or from a differentmagmatic source. This bimodal volcanism predatedand differs from the alkaline volcanism thatoccurred bordering the neighbouring Bou-Azzerinlier, indicating the presence of different magmaticsources associated with a rifting that postdatedthe Pan-African orogeny and ended in MiddleCambrian times.

The authors acknowledge the numerous useful remarksmade by U. Linnemann and A. C. Maalouf, which havehelped to improve the ideas expressed in this paper.Research on the Adoudou volcano-sedimentary complexhas been supported by ECLIPSE project ‘Evolution of bio-geochemical cycles from Archean to Recent environ-ments’, and GRICES/CNRST project ‘A comparativestudy of the Neoproterozoic–Cambrian transitionbetween the Portuguese Ossa–Morena Zone and theMoroccan Anti-Atlas and Meseta: geologic and geochem-ical aspects, and geodynamic model’. This paper is a con-tribution to CGL2006-13533 Programme and IGCPproject 485 ‘Cratons, metacratons and mobile belts: keysfrom the West African craton boundaries, Eburneanversus Pan-African signature, magmatic, tectonic andmetallogenic implications’.

References

AIT AYAD, N., RIBEIRO, M. L., MATA, J., FERREIRA, P.,EZZOUHAIRI, H., CHARIF, A. & DIAS, R. 1998.Evolution du magmatisme cambrien en deux regionsperigondwanniennes: Azegour (Haut-Atlas) et Alterdo Chao-Elvas (NE Alentejo). Communicacaosdo Instituto Geologico e Mineiro do Portugal, 84,B154–B157.

AL-ASSAM, I., TAYLOR, B. E. & SOUTH, B. 1990. Stableisotope analysis of multiple carbonate samples usingselective acid extraction. Chemical Geology, 80,119–125.

ALVARO, J. J. & CLAUSEN, S. 2006. Microbial crusts asindicators of stratigraphic diastems in the CambrianMicmacca Breccia, Moroccan Atlas. SedimentaryGeology, 185, 255–265.

ALVARO, J. J. & CLAUSEN, S. 2007. Botoman(Lower Cambrian) turbid- and clear-water reefs andassociated environments from the High Atlas,Morocco. In: ALVARO, J. J., ARETZ, M., BOULVAIN,F., MUNNECKE, A., VACHARD, D. & VENNIN, E.(eds) Palaeozoic Reefs and Bioaccumulations:Climatic and Evolutionary Controls. GeologicalSociety, London, Special Publications, 275, 51–70.

ALVARO, J. J. & CLAUSEN, S. 2008. Paleoenvironmentalsignificance of hiatal shelled accumulations in aCambrian intracratonic aborted rift, Atlas Mountains,Morocco. Geological Association of Canada, SpecialPaper on Epeiric Seas (in press).

ALVARO, J. J., CLAUSEN, S., EL ALBANI, A. &CHELLAI, E. H. 2006a. Facies distribution ofLower Cambrian cryptic microbial and epibenthicarchaeocyathan–microbial communities in the


western Anti-Atlas, Morocco. Sedimentology, 53,35–53.

ALVARO, J. J., EZZOUHAIRI, H., VENNIN, E. ET AL.2006b. The Early Cambrian Boho volcano of the ElGraara massif, Morocco: petrology, geodynamicsetting and coeval sedimentation. Journal of AfricanEarth Sciences, 44, 396–410.

AMTHOR, J., GROTZINGER, J., SCRODER, S., BOWRING,S., RAMEZANI, J., MARTIN, M. & MATTER, A. 2003.Extinction of Cloudina and Namacalathus at thePrecambrian–Cambrian boundary in Oman.Geology, 31, 431–434.

BENSSAOU, M. 2005. Le Rift Cambrien inferieur del’Anti-Atlas, Maroc: remplissage sedimentaire, geo-dynamique et paleogeographie. These Doctoratd’Etat, Universite Ibn Zohr, Agadir.

BENSSAOU, M. & HAMOUMI, N. 2001. L’Anti-Atlasoccidental du Maroc: etude sedimentologique etreconstitutions paleogeographiques au Cambrieninferieur. Journal of African Earth Sciences, 32,351–372.

BERGER, R. A. 1990. Atmospheric carbon dioxide overPhanerozoic time. Science, 249, 1382–1386.

BERTRAND, H. 1975. Les dolerites marocaines et l’ouver-ture de l’Atlantique: etude petrologique et geochimi-que. These 3eme cycle, Universite de Lyon.

BOUDDA, A., CHOUBERT, G. & FAURE-MURET, A.1979. Essai de stratigraphie de la couverture sedimen-taire de l’Anti-Atlas: Adoudounien, Cambrien infer-ieur. Notes et Memoires du Service Geologique duMaroc, 271, 1–96.

BUGGISCH, W. & FLUGEL, E. 1988. The Precambrian/Cambrian boundary in the Anti-Atlas (Morocco).Discussion and new results. In: JACOBSHAGEN,V. H. (ed.) The Atlas System of Morocco. Studies onits Geodynamic Evolution. Lecture Notes in EarthSciences, 15, 81–90.

BUGGISCH, W. & HEINITZ, W. 1984. Slumpfolds andother early deformations in the early Cambrian ofthe western and central Anti-Atlas (Morocco).Geologische Rundschau, 73, 809–818.

BUGGISCH, W., MARZELA, C. & HUGEL, P. 1978. Diefazielle und palaogeographische Entwicklung derinfrakambrischen bis ordovizischen Sedimente imMittleren Antiatlas um Agdz (S-Marokko). Geolo-gische Rundschau, 68, 195–224.

CHAZAN, W. 1954. Les gisements stratiformes plombo-zinciferes de l’Infracambrien de l’Anti-Atlas occi-dental (Maroc). Notes et Memoires du ServiceGeologique du Maroc (VIII), 120, 97–126.

CHBANI, B., BEAUCHAMP, J., ALGOUTI, A. & ZOUHAIR,A. 1999. Un enregistrement sedimentaire eocambriendans un bassin intracontinental en distension: lecycle ‘conglomerats de base–unite calcaire–gres deTikirt’ de Bou-Azzer El Graara (Anti-Atlas central,Maroc). Comptes Rendus de l’Academie des Sciences,329, 317–323.

CHOUBERT, G. 1952. Histoire geologique du domaine del’Anti-Atlas. In: CHOUBERT, G. & MARCAIS, J. (eds)Geologie du Maroc. XIX Congres Geologique Inter-national d’Alger. Monographies Regulieres (Serie 3,Maroc), 6, 77–194.

CHOUBERT, G. & FAURE-MURET, A. 1970. Livret-guidede l’excursion Anti-Atlas occidental et central. Noteset Memoires du Service Geologique du Maroc, 299.

COMPSTON, W., WILLIAMS, J. L., KIRSCHVINK, J. L.,ZHANG, Z. & MA, G. 1992. Zircon U–Pb ages forthe Early Cambrian time scale. Journal of the Geologi-cal Society, London, 127, 319–332.

DEMANGE, M. 1980. Stratigraphie, volcanisme et paleo-geographie du Precambrien III et de la serie de basedans la partie sud de la boutonniere d’Ouaoufenrha(Anti-Atlas occidental). Mineralisations associees.Notes et Memoires du Service Geologique du Maroc,285, 7–23.

DESTOMBES, J., HOLLARD, H. & WILLEFERT, S. 1985.Lower Palaeozoic rocks of Morocco. In: HOLLAND,C. H. (ed.) Lower Palaeozoic Rocks of the World.Vol. 4. Lower Palaeozoic of North-Western and WestCentral Africa. Wiley, Chichester, 157–184.

DOSTAL, J., DUPUY, C. & BARAGAR, W. R. A. 1983.Geochemistry and petrogenesis of basaltic rocksfrom Coppermine River area, Northwest Territories.Canadian Journal of Earth Sciences, 20, 684–698.

DUCROT, J. & LANCELOT, J. R. 1977. Probleme de lalimite Precambrien–Cambrien: etude radiochrono-logique par la methode U/Pb sur zircon du volcandu Jbel Boho. Canadian Journal of Earth Sciences,14, 1771–1777.

DUPUY, C. & DOSTAL, J. 1984. Trace element geochem-istry of some continental tholeiites. Earth and Plane-tary Science Letters, 67, 61–69.

EZZOUHAIRI, H. 2001. Le magmatisme post-collisionnelpanafricain (tardi a post-orogenique) des regionsd’Aghbalou, Sidi Flah-Bouskour et Oued Imini(Ouarzazate, Anti-Atlas central, Maroc). Lithostratigra-phie, geochimie, petrogenese et contexte geodynamique.PhD thesis, University Chouaıb Doukkali, El Jadida.

EZZOUHAIRI, H., RIBEIRO, M. L., CHARIF, A., AIT

AYAD, N., RAMOS, F., MOREIRA, M. E. & COKE,C. 2003. Contribution a la caracterisation petrographi-que et geochimique du volcanisme cambrien moyendu mole cotier (Maroc). In: REMAL, T. (ed.) 3rdInternational Colloquium on Magmatism, Metamorph-ism and Associated Mineralization, University HassanII - Mohammadia, Casablanca, 8–10 Mai, Resumes,49–50.

EZZOUHAIRI, H., RIBEIRO, M. L., AIT AYAD, N. ET AL.2008. The magmatic evolution at the Moroccan out-board of the West African Craton between the LateNeoproterozoic and the Early Palaeozoic. In: ENNIH,N. & LIEGEOIS, J. -P. (eds) The Boundaries of theWest African Craton. Geological Society, London,Special Publications, 297, 329–343.

GASQUET, D., LEVRESSE, G., CHEILLETZ, A., RACHID

AZIZI-SAMIZ, M. & MOUTTAQI, A. 2005.Contribution to a geodynamic reconstruction ofthe Anti-Atlas (Morocco) during Pan-Africantimes with the emphasis on inversion tectonics andmetallogenic activity at the Precambrian–Cambriantransition. Precambrian Research, 140, 157–182.

GEYER, G. & LANDING, E. (eds) 1995. Morocco’95. TheLower–Middle Cambrian standard of Gondwana.Beringeria, Special Issue, 2, 1–171.

HARKER, A. 1909. The Natural History of Igneous Rocks.Hafner, New York (1965 facsimile of original edition).

HELG, U., BUKHARD, M., CARITG, S. &TOBERT-CHARRUE, C. 2004. Folding and inversiontectonics in the Anti-Atlas of Morocco. Tectonics,21, TC4006, doi:10.1029/2003TC001576.


HOUZAY, J. P. 1979. Empreintes attribuables a desmeduses dans la serie de base de l’Adoudounien(Precambrien terminal de l’Anti-Atlas, Maroc).Geologie Mediterraneenne, 6, 379–384.

KIRSHVINK, J. L., MAGARITZ, M., RIPPERDAN, R. L. &ZHURAVLEV, A. YU. 1991. The Precambrian/Cambrian boundary: Magnetostratigraphy and carbonisotopes resolve correlation problems between Siberia,Morocco, and South China. GSA Today, 1, 69–71.

LANDING, E., BOWRING, S. A., DAVIDEK, K. L.,WETROP, S. R., GEYER, G. & HELDMAIER, W.1998. Duration of the Early Cambrian: U–Pb ages ofvolcanic ashes from Avalon and Gondwana. CanadianJournal of Earth Sciences, 35, 329–338.

LATHAM, A. & RIDING, R. 1990. Fossil evidence of thelocation of the Precambrian/Cambrian boundary inMorocco. Nature, 344, 752–754.

MAALOUF, A. C., SCHRAG, D. P., CROWLEY, J. L. &BOWRING, S. A. 2005. An expanded record of EarlyCambrian carbon cycling for the Anti-Atlas margin,Morocco. Canadian Journal of Earth Sciences, 42,2195–2216.

MAGARITZ, M., KIRSHVINK, J. L., LATHAM, A. J.,ZHURAVLEV, A. YU. & ROZANOV, A. YU. 1991.Precambrian/Cambrian boundary problem: Carbonisotope correlations for Vendian and Tommotian timebetween Siberia and Morocco. Geology, 19, 847–850.

MARSH, J. S. 1987. Basalt geochemistry and tectonic dis-crimination within continental flood basalt provinces.Journal of Volcanology and Geothermal Research,32, 35–49.

MIYASHIRO, A. & SHIDO, F. 1975. Tholeiitic andcalc-alkaline series in relation to the behaviors of Ti–Cr–Ni. American Journal of Science, 275, 265–277.

PEARCE, J. A. 1983. Role of the sub-continental litho-sphere in magma genesis at active continentalmargins. In: HAWKESWORTH, C. J. & NORRY, M. J.(eds) Continental Basalts and Mantle Xenoliths.Shiva, Nantwich, 230–249.

PEARCE, J. A. & GALE, G. H. 1977. Identificationof ore-deposition environment from trace elementgeochemistry of associated igneous host rocks. In:FYFE, W. S. (ed.) Volcanic Processes in Ore Deposits.Geological Society, London, Special Publications, 7,14–24.

PIQUE, A., BOUABDELLI, M. & DARBOUX, J. R. 1995.Le rift cambrien du Maroc occidental. ComptesRendus de l’Academie des Sciences, 320, 1017–1024.

PRATT, B. R., JAMES, N. P. & CLINTON, A. C. 1992.Peritidal carbonates. In: WALKER, R. G. & JAMES,N. P. (eds) Facies Models: Response to Sea LevelChange. Geological Association of Canada, Ottawa,Ont., 303–322.

SCHLISCHE, R. W. 1992. Structural and stratigraphicdevelopment of the Newark extensional basin,eastern North America: Evidence for the growth ofthe basin and its bounding structures. GeologicalSociety of American Bulletin, 104, 1246–1263.

SCHMITT, M. 1979. The section of Tiout (Precambrian/Cambrian boundary beds, Anti-Atlas, Morocco):stromatolites and their biostratigraphy. Arbeiten ausdem Palaontologischen Institut Wurzburg, 2, 1–188.

SDZUY, K. 1978. The Precambrian–Cambrian boundarybeds in Morocco (preliminary report). GeologicalMagazine, 115, 83–94.

SERVICE GEOLOGIQUE DU MAROC 1970. Carte geologi-que de l’Anti-Atlas central et de la zone synclinale deOuarzazate. Feuilles Ouarzazate, Alougoum et TelouetSud, 1/200 000. Notes et Memoires du Servicegeologique du Maroc, 138.

SOULAIMANI, A., PIQUE, A. L. & BOUABDELLI, M.2001. La serie du PII–PIII de l’Anti-Atlas occidental(Sud marocain): un olistostrome a la base de la couver-ture post-panafricaine (PIII) du Proterozoıque super-ieur. Comptes Rendus de l’Academie des Sciences,332, 121–127.

SOULAIMANI, A., BOUABDELLI, M. & PIQUE, A. L.2003. L’extension continentale au Neoproterozoıquesuperieur-Cambrien inferieur dans l’Anti-Atlas(Maroc). Bulletin de la Societe Geologique deFrance, 174, 83–92.

SOULAIMANI, A., ESSAIFI, A., YOUBI, N. & HAFID, A.2004. Les marqueurs structuraux et magmatiquesde l’extension crustale au Proterozoıque terminal–Cambrien basal autour du massif de Kerdous(Anti-Atlas occidental, Maroc). Comptes Rendus del’Academie des Sciences, 336, 1433–1441.

SUN, S. S. & MCDONOUGH, W. F. 1989. Chemical andisotopic systematics of oceanic basalts: implicationsfor mantle composition and processes. In: SAUNDERS,A. D. & NORRY, M. J. (eds) Magmatism in theOceanic Basins. Geological Society, London, SpecialPublications, 42, 313–345.

SWART, P. K., BURNS, S. J. & LEDER, J. J. 1991. Frac-tionation of the stable isotopes of oxygen and carbonin carbon dioxide during the reaction of calcite withphosphoric acid as a function of temperature andtechnique. Chemical Geology, Isotopic GeosciencesSection, 86, 89–96.

TOBISCH, O. 1984. Development of foliation and foldinterference patterns produced by sedimentary pro-cesses. Geology, 12, 441–444.

TUCKER, E. 1986. Carbon isotope excursions inPrecambrian/Cambrian boundary beds, Morocco.Nature, 319, 48–50.

VAN LOOY, J. 1985. Het kaartblad Tazenakht 1/100.000,Anti-Atlas, Marokko. Kartiering, Lithostratigrafie,Biostratigrafie van Precambrium tot Tremadoc. PhDthesis, Katholieke Universiteit, Leuven.

VILAND, J. C. 1977. Presence d’horizons phosphates a labase de l’Infracambrien superieur du Haut Atlas deMarrakech (Maroc). Notes et Memoires du Servicegeologique du Maroc, 268, 13–22.

WILSON, M. 1994. Igneous Petrogenesis. A GlobalTectonic Approach. Harper Collins, London.

WOOD, D. A. 1980. The application of the Th–Hf–Tadiagram to problems of tectono-magmatic classifi-cation, and to establishing the nature of the crustalcontamination of basaltic lavas of the British TertiaryProvince. Earth and Planetary Science Letters, 50,11–30.

YUI, T. F. & GONG, S. Y. 2003. Stoichiometry effecton stable isotope analysis of dolomite. ChemicalGeology, 201, 359–368.


Documents

Late Neoproterozoic carbonate productivity in a rifting context: the Adoudou Formation and its associated bimodal volcanism onlapping the western Saghro inlier, …